Wiring substrates of this type are used for semiconductor component housings, also known as BGA housings (ball grid array) or as LBGA housings (large ball grid array).
One of the problems of the wiring substrates is the reliability with which a connection between the external contact pads of the wiring substrate and a superordinate circuit board can be produced. In the event of cyclic thermal loadings, cyclic mechanical shear loadings are exerted on the solder connections on account of the presence of different materials having different coefficients of thermal expansion. In the event of vibration loadings, pressure fluctuations occur in addition to the shear loadings of the cyclic thermal stresses, and may in total lead to microcracks or to the failure of the connection.
Many different methods are used in the prior art in order to improve the reliability of the solder ball connections to superordinate circuit boards. Some methods are based on the configuration of the solder connections in order to reduce the mechanical loading. Other methods attempt to adapt the material properties of the housing packages in such a way that the mechanical loadings are reduced, such as for example by inserting semiconductor chip adhesive layers. Some of these solutions have the disadvantage that the adaptation of the housing substrates is very greatly dependent on the specific embodiment of the housings. These solutions require additional complex and expensive fabrication methods. These problems are also exasperated if a transition is made to smaller distances between the soldering contacts and to smaller connection pitches.
Moreover, a transition from lead-containing to lead-free solder materials, on account of different properties of the respective materials, will be more likely to increase rather than reduce the problems with regard to the loading limits of solder balls as external contacts. Further, possibilities for solving the problems involve introducing an additional material in the form of an underfiller between the wiring substrate of the housing and the superordinate circuit board. However, this requires an additional process step during the production of circuit boards and makes it more difficult to exchange or repair assemblies on the circuit boards.
Further solution approaches in the prior art provide for embodying the contacts themselves in flexible fashion, e.g., via the external contacts forming spring elements or being embodied from flexible materials in the form of elastomer balls with rubber-elastic cores. However, these variants with flexible contacts are highly cost-intensive and unsuitable for mass production. Moreover, reliability problems arise due to the low degree of robustness of the spring elements or elastomer balls as external contacts.
Flip-chip components, semiconductor chip size housings, and BGA housings have been proposed, which have solder balls which are arranged over an air-filled cavity or are arranged over a cavity filled with an elastic material. So-called “floating pad” solder balls of this type require a very complex cost-intensive fabrication method based on a specific military application and are therefore likewise not suitable for mass production. In principle, the cavities or the elastomer-filled cavities are realized by application of additional complex layers to the wiring substrate.